Nitride semiconductor light-emitting device and method for producing same

ABSTRACT

In a method for producing a nitride semiconductor light-emitting device according to the present invention, first, a nitride semiconductor substrate having groove portions formed is prepared. An underlying layer comprising nitride semiconductor is formed on the nitride semiconductor substrate including the side walls of the groove portions, in such a manner that the underlying layer has a crystal surface in each of the groove portions and the crystal surface is tilted at an angle of from 53.5° to 63.4° with respect to the surface of the substrate. Over the underlying layer, a light-emitting-device structure composed of a lower cladding layer containing Al, an active layer, and an upper cladding layer containing Al is formed. According to the present invention, thickness nonuniformity and lack of surface flatness, which occur when accumulating a layer with light-emitting-device structure of nitride semiconductor over the nitride semiconductor substrate, are alleviated while inhibiting occurrence of cracking.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing a nitridesemiconductor light-emitting device, and more particularly to a methodfor producing a nitride semiconductor light-emitting device improved toprevent cracking and obtain nitride semiconductors and nitridesemiconductor laser devices at a high yield rate. The present inventionalso relates to a nitride semiconductor light-emitting device obtainedby such a method.

By using nitride semiconductor crystals represented by GaN, AlN, InN,and mixed crystals thereof, semiconductor laser devices that oscillatein the ultraviolet-visible region are being produced. As the substrate,a GaN substrate is often used and being studied with vigor in associatedresearch institutions. Currently, the yield rate of semiconductor laserdevices (e.g., the rate of good products obtained from one wafer) issignificantly low, and improvement is highly necessary for costreduction and the like. One of the causes that have kept the yield ratelow is the occurrence of cracking.

As a technique to reduce occurrence of cracking, a method of using aprocessed substrate is proposed as shown in FIG. 6 (see, for example,Japanese Patent Application Publication No. 2002-246698). Referring toFIG. 6(A), processed substrate 61 (nitride semiconductor substrate)includes grooves 17 that are concave portions each processed in a stripeshape on the substrate surface. Over hills 18, which are convex portionseach processed in a stripe shape, a light-emitting device made ofnitride semiconductor is formed. It is known that by using suchprocessed substrate 61 concaves are included in the surfaces of thesemiconductor films that have been grown, thereby inhibiting occurrenceof cracking in the light-emitting device.

It is also known that cracking is reduced by using a processed substrateshown in FIG. 6(B). This processed substrate has different kindsubstrate 62 (sapphire substrate, SiC substrate, Si substrate, GaAssubstrate, or the like) and nitride semiconductor layer 64 formed overdifferent kind substrate 62 via buffer layer 63 (nitride semiconductorlayer (of low temperature or high temperature)). On the surface ofnitride semiconductor layer 64, grooves 17 are formed each in a stripeshape. Over hills 18, which are convex portions each processed in astripe shape, a light-emitting device made of nitride semiconductor isformed.

However, when the above-described processed substrate was used andnitride compound semiconductors were grown over the substrate by theMOCVD (Metal Organic Vapor Deposition) method or the like to prepare asemiconductor laser device, occurrence of cracking was prevented, but nogreat improvement in the yield rate was obtained.

The prevent inventors carried out an extensive study to analyze thecause of why the yield rate did not improve greatly. As a result, asshown in FIG. 7, it has been found that nitride semiconductor layers(those in the areas 130 shown in FIG. 7) accumulated so as to cover fromthe side surfaces of the groove portions (the portions where groovesportions 17 and hill portions 18 were in contact) to hill portions 18have undulating-shaped surface morphologies 140 and are grown muchthicker than at the center portions of hill portions 18. (This growthwill be hereinafter referred to as abnormal growth.) In addition, thesizes and thicknesses of abnormal growth portions 130, caused byabnormal growth, were different between adjacent groove portions 17, andalong the stripe of the groove portion 17 even if it was the same grooveportion 17. It has been found that if abnormal growth portion 130 is notformed uniformly in each groove portion 17 as described above, theformation of undulating-shaped surface morphology 140 is promoted in thevicinity of the edges of groove portion 17, and that the promotedsurface morphology 140 undermines the thickness uniformity and flatnessof the nitride semiconductor layers in the vicinity of the center ofhill portion 18. It is considered that occurrence of such a phenomenonis because when grooves 17 are formed on the nitride semiconductorsubstrate by dry etching or wet etching, the edge portions of the sidesof groove portions 17 cannot be made uniform and take various shapes.

Thus, if concaves (grooves) remain on a nitride compound semiconductor,the concaves cause deterioration of the flatness of the film. It isconsidered that the deteriorated flatness in turn causes variation ofthickness of each layer of the device and chip-to-chip fluctuation ofcharacteristics, resulting in a deteriorated yield rate. In other words,in order to improve the yield rate, it is necessary to improve filmflatness as well as reducing occurrence of cracking.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forproducing a nitride semiconductor light-emitting device improved toprevent occurrence of cracking and produce a nitride semiconductorcompound film that has high flatness.

It is another object of the present invention to provide a method forproducing a nitride semiconductor light-emitting device improved toobtain a semiconductor laser device at a high yield rate.

It is another object of the present invention to provide a method forproducing a nitride semiconductor light-emitting device improved toobtain a highly reliable semiconductor laser device.

It is another object of the present invention to provide a nitridesemiconductor light-emitting device obtained by the above method.

In a method for producing a nitride semiconductor light-emitting deviceaccording to the present invention, first, a nitride semiconductorsubstrate having stripe groove portions is prepared. An underlying layercomprising nitride semiconductor is formed on the nitride semiconductorsubstrate including the side walls of the groove portions, in such amanner that the underlying layer has a crystal surface in each of thegroove portions and the crystal surface is tilted at an angle of from53.5° to 63.4° with respect to the surface of the substrate. Over theunderlying layer, a lower cladding layer containing Al, an active layer,and an upper cladding layer containing Al are sequentially formed.

With this invention, since a crystal surface that is tilted at an angleof from 53.5° to 63.4° with respect to the substrate surface is formedin each of the groove portions, and a layer of light-emitting-devicestructure made of nitride semiconductor is formed over the substratewhile maintaining the crystal surface, the uniformity and flatness ofthe layer of light-emitting-device structure accumulated over the hillportion between the groove portions are improved. By forming a laserwaveguiding structure over the layer of light-emitting-device structureformed in the region of the hill portion, variation of characteristicsbetween nitride semiconductor light-emitting devices (chips) is reduced,resulting in an improved yield rate of nitride semiconductorlight-emitting devices.

In a preferred embodiment of the present invention, the compositionratio of Al of the underlying layer is lower than the composition ratioof Al of the lower cladding layer. More preferably, the compositionratio of Al in the underlying layer is 5% or lower.

This is because if such an underlying layer is formed that has the sameAl composition ratio as or higher Al composition ratio than that of thelower cladding layer, the shapes of the groove portions are maintained(that is, no crystal surface is formed), and unless the underlying layeris grown considerably thick, the crystal surface is not formed in thevicinity of the side surface of each groove portion. Since such atendency becomes notable as the composition ratio of Al contained in theunderlying layer becomes higher, the composition ratio of Al containedin the underlying layer is preferably 5% or lower. Use of such anunderlying layer that does not contain Al and is made of GaN isparticularly preferable in that the accumulation thickness of theunderlying layer is thin and the crystal surface is formed with ease.

The present invention is characterized in that the crystal surfacecomprises a {11-22} surface.

Since the {11-22} surface is stable and a rather flat surface, theuniformity of thickness of the nitride semiconductor layers in thevicinity of the center of the hill portion is realized. Further,formation of the above-described undulating-shaped surface morphologythat undermines flatness is prevented satisfactorily.

After the crystal surface is formed by accumulation of the underlyinglayer, the crystal surface is maintained through subsequent accumulationof the underlying layer. However, as the thickness of the underlyinglayer increases, the grooves are covered with nitride semiconductor(underlying layer) and the area of the crystal surface formed in thevicinity of the side surface of each groove gradually decreases, andconsequently, the grooves are completely filled up and the crystalsurface disappears. In view of this, the thickness of the underlyinglayer needs to be limited such that the grooves are not completelyfilled up (that is, the crystal surface does not disappear). Preferably,the thickness of the underlying layer is from 0.01 μm to 2 μm.

The depth of each of the groove portions may be from 0.5 μm to 20 μm,and more preferably from 0.5 μm to 8 μm. If the depth of each of thegrooves is less than 0.5 μm, although the crystal surface appears aftercommencement of growth of the underlying layer, the grooves are filledup in the course of growth and the crystal surface cannot be maintained,which is not preferable. As the groove becomes deeper, surface flatnessdeteriorates, and therefore, the groove depth Z is preferably 8 μm orless. On the other hand, if the depth Z of each of the groove portionsexceeds 20 μm, cracking of the wafer occurs in the production process ofthe nitride semiconductor laser device, which is not preferable. Byselecting the depth of the groove portions in this manner, the grooveportions are prevented from being filled up by nitride semiconductor,and consequently, the crystal surface is prevented from disappearing.

The width of each of the grooves is preferably from 5 μm to 100 μm. Thisis because if the width of each of the grooves is less than 5 μm, thegrooves are filled up by nitride semiconductor (underlying layer) andthe crystal surface disappears, and thus improvement of flatness anduniformity of the nitride semiconductor layers cannot be expected. Ifthe width of the grooves exceeds 100 μm, the number of nitridesemiconductor light-emitting devices (chips) obtained from one wafer isreduced, which is not preferable.

A nitride semiconductor light-emitting device according to anotheraspect of the present invention comprises a nitride semiconductorsubstrate. On the nitride semiconductor substrate, an underlying layermade of nitride semiconductor is provided. On the underlying layer, alower cladding layer containing Al, an active layer, and an uppercladding layer containing Al are sequentially provided.

By providing an underlying layer made of nitride semiconductor on thenitride semiconductor substrate, the uniformity and flatness of thelayer of light-emitting-device structure accumulated over the hillportion between the groove portions are improved, as described above. Byforming a laser waveguiding structure over the layer oflight-emitting-device structure formed in the region of the hillportion, variation of characteristics between nitride semiconductorlight-emitting devices (chips) is reduced, resulting in an improvedyield rate of nitride semiconductor light-emitting devices.

The composition ratio of Al of the underlying layer is preferably lowerthan the composition ratio of Al of the lower cladding layer. Morepreferably, the composition ratio of Al of the underlying layer is 5% orlower. More preferably, the underlying layer comprises GaN.

The thickness of the underlying layer is preferably from 0.01 μm to 2μm.

By using the present invention, nitride semiconductor grown layers withgood surface flatness and inhibited cracking are formed over the nitridesemiconductor substrate. This enables it to obtain highly reliablenitride semiconductor light-emitting devices at a high yield rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) is a view showing the C surface of a hexagonal crystal, andFIG. 1(B) is a view showing the coordinate system of a hexagonalcrystal.

FIG. 2 is a view showing the upper surface of the nitride semiconductorsubstrate before formation of the underlying layer.

FIG. 3 is a cross sectional view of the nitride semiconductor substrateon which the underlying layer is formed.

FIG. 4 is a schematic cross sectional view of a part of a bar providedwith a nitride semiconductor laser device of an embodiment of thepresent invention.

FIG. 5 is a schematic cross sectional view of the vicinity of the hillportion shown in FIG. 4.

FIG. 6 is a cross sectional view of a conventional processed substrate.

FIG. 7 is a view for showing a problem associated with use of theconventional processed substrate.

In the figures, reference numeral 10 denotes a nitride semiconductorsubstrate, 11 denotes a layer of light-emitting-device structure made ofnitride semiconductor, 12 denotes a ridge stripe portion, 13 denotes aninsulation film, 14 denotes a p-electrode, 15 denotes an n-electrode, 16denotes a crystal surface, 17 denotes a groove portion (groove), 18denotes a hill portion (hill), 21 denotes an underlying layer made ofnitride semiconductor, 22 denotes an n-type Al_(0.062)Ga_(0.938)N firstcladding layer, 23 denotes an n-type Al_(0.1)Ga_(0.9)N second claddinglayer, 24 denotes an n-type Al_(0.062)Ga_(0.938)N third cladding layer,25 denotes an n-type GaN light guiding layer, 26 denotes an active layerof multi-quantum well structure, 27 denotes a p-type Al_(0.3)Ga_(0.7)Ncarrier blocking layer, 28 denotes a p-type GaN light guiding layer, 29denotes a p-type Al_(0.1)Ga_(0.9)N cladding layer, 30 denotes a p-typeGaN contact layer, 41 denotes a dividing line, 100 denotes the normaldirection of the nitride semiconductor substrate, 101 denotes the normaldirection of the underlying layer, 110 denotes the normal direction ofthe crystal surface, 120 denotes the angle between the nitridesemiconductor substrate or the underlying layer and the crystal surface,130 denotes a portion of abnormal growth, and 140 denotesundulating-shaped surface morphology.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preparation of nitride semiconductor grown layers with good surfaceflatness and inhibited cracking over a nitride semiconductor substrate,which is an object of the present invention, has been realized bypreparing a nitride semiconductor substrate having stripe grooveportions, and by forming an underlying layer having nitridesemiconductor on the nitride semiconductor substrate including the sidewalls of the groove portions, in such a manner that the underlying layerhas a crystal surface in each of the groove portions and the crystalsurface is tilted at an angle of from 53.5° to 63.4° with respect to thesurface of the substrate.

First, the meanings of terms used in this specification will beclarified in advance.

The term “nitride semiconductor substrate” can be any substrate that ismade of nitride semiconductor, examples including a Al_(a)Ga_(b)In_(c)N(0≦a≦1, 0≦b≦1, 0≦c≦1, a+b+c=1) substrate. It is also possible thatapproximately 10% or less of the nitrogen element (of hexagonal crystalsystem) in the Al_(a)Ga_(b)In_(c)N (0≦a≦1, 0≦b≦1, 0≦c≦1, a+b+c=1)substrate is replaced with any one of the elements As, P, and Sb.

It is also possible that the nitride semiconductor substrate is dopedwith Si, O, Cl, S, C, Ge, Zn, Cd, Mg, or Be. For n-type nitridesemiconductor, Si, O, and Cl are particularly preferable among the abovedoping materials. Referring to FIG. 1(A), the main surface orientationof the nitride semiconductor substrate is most preferably the C surface{0001}. If the substrate main surface has an off angle (tilt of thesubstrate surface from the C surface) of 2° or less with respect to theC surface, the surface morphology is satisfactory. Further, if the offangle is in a parallel direction of the groove to be prepared, the shapeof the side surfaces of the groove is stabilized, resulting in moresatisfactory surface morphology. FIG. 1(B) shows the coordinate systemof a hexagonal crystal. Examples of the nitride semiconductor substratepreferably used in the present invention include a GaN substrate, an AlNsubstrate, and an AlGaN substrate.

The term “groove portion” or “groove” as used in this specificationdenotes a stripe concave portion on the upper surface (growth surface)of the nitride semiconductor substrate. The term “hill portion” or“hill” denotes a stripe convex portion between a groove and a groove. Itshould be noted that the cross sectional view of each of the groove andhill is not necessarily rectangular, and for example, a forward taperedshape or an inverse tapered shape can be used.

The term “side surface of the groove portion” as used in thisspecification denotes the side surface shared by the groove and hill asa result of the digging and forming of the groove.

The term “active layer” as used in this specification denotes a generalterm for a layer composed of a well layer or well layers and barrierlayers. For example, an active layer of single quantum well structure iscomposed of one well layer or barrier layer/well layer/barrier layer. Anactive layer of multi-quantum well structure is composed of a pluralityof well layers and a plurality of barrier layers.

Although it is common practice in crystallography to draw a line abovethe absolute value to indicate the case where the index of the surfaceor orientation of the crystal is negative, such a way of indication isimpossible in this specification. Therefore, the negativity of the indexwill be indicated by the negative sign “−” placed in front of theabsolute value, e.g., the {11-22} surface and the <1-100> direction.

An embodiment of the present invention will be described below byreference to the drawings. First, preferred conditions for forming theunderlying layer, which are features of the present invention, will bedescribed, followed by a description of a nitride light-emitting deviceformed over the substrate including the underlying layer.

1) Formation of Underlying Layer

FIG. 2 is a view showing the upper surface of the nitride semiconductorsubstrate before formation of the underlying layer. The surfaceorientations are also shown. Referring to FIG. 2, stripe groove portions17 are formed on the surface of nitride semiconductor substrate 10(e.g., an n-type GaN substrate). Referring to FIG. 3 (a cross sectionalview of the nitride semiconductor substrate in the vicinity of a grooveportion), underlying layer 21 made of nitride semiconductor isaccumulated on nitride semiconductor substrate 10 including the sidewalls of groove portion 17. By accumulating underlying layer 21, in thevicinity of the side surfaces of groove portion 17, crystal surfaces 16each tilted at an angle of from 53.5° to 63.5° with respect to thesurface of nitride semiconductor substrate 10 are formed.

By using FIG. 3, the angle of crystal surface 16 according to thepresent invention will be defined. Crystal surface 16 that is tilted atan angle of from 53.5° to 63.5° with respect to the surface of nitridesemiconductor substrate 10 means such a crystal surface that angle 120between normal direction 100 of nitride semiconductor substrate 10 (ornormal direction 101 of layer 21 of light-emitting-device structure madeof nitride semiconductor in the vicinity of the hill portion) and normaldirection 100 of the crystal surface is 53.5° or greater and 63.5° orless. This crystal surface 16 is so stable that once it is formed in thevicinity of the side surface of groove portion 17, transition to anothercrystal surface is hard to occur. This means that the shapes of thevicinities of the side surfaces of groove portions 17 (i.e., the shapesof the edges of the groove portions) become approximately uniform overthe entire regions of groove portions 17 formed on nitride semiconductorsubstrate 10 (i.e., the edges of groove portions 17 are occupied bycrystal surfaces 16).

If the shapes of the vicinities of the side surfaces of groove portions17 become uniform, undulating-shaped surface morphology is inhibited asdescribed later, and uniformity and flatness of the thickness of thenitride semiconductor layers in the vicinity of the center of the hillportion (layer 11 of light-emitting-device structure made of nitridesemiconductor shown in FIG. 4, described later) are considered to berealized.

The composition ratio of Al in underlying layer 21, which is made ofnitride semiconductor and formed on nitride semiconductor substrate 10on which stripe groove portions 17 are formed, is preferably lower thanthe composition ratio of Al in the lower cladding layer. Since the lowercladding layer confines light from the light-emitting layer, the Alcomposition ratio needs to be generally made high. Here the lowercladding layer or upper cladding layer is, specifically, AlGaN andInAlGaN, and the Al composition ratio is preferably from 0.04 to 0.15.This is because if such an underlying layer is formed that has the sameAl composition ratio as or higher Al composition ratio than that of thelower cladding layer, then underlying layer 21 is formed such that theshape of groove portion 17 (formed on nitride semiconductor substrate 10by the dry etching method or wet etching method) is maintained (i.e., nocrystal surface is formed), or unless underlying 21 is grownconsiderably thick, the above-described crystal surface 16 is not formedin the vicinity of the side surface of groove portion 17. Since such atendency becomes notable as the composition ratio of Al contained in theunderlying layer becomes higher, the composition ratio of Al containedin the underlying layer is preferably 5% or lower. Use of such anunderlying layer that does not contain Al and is made of GaN isparticularly preferable in that the accumulation thickness of theunderlying layer is thin and the crystal surface is formed with ease.

Crystal surface 16 preferably is a surface containing the {11-22}surface. Since the {11-22} surface is stable and a rather flat surface,the uniformity of thickness of the nitride semiconductor layers in thevicinity of the center of the hill portion is realized, and further,formation of the above-described undulating-shaped surface morphology140 (see FIG. 7) that undermines flatness is prevented satisfactorily.

After the above-described crystal surface 16 is formed by accumulationof underlying layer 21, crystal surface 16 is maintained throughsubsequent accumulation of the underlying layer. However, as thethickness of underlying layer 21 increases, the inside in the grooveportion is covered with nitride semiconductor (underlying layer) and thearea of the crystal surface formed in the vicinity of the side surfaceof groove portion 17 gradually decreases, and consequently, grooveportion 17 is completely filled up and the crystal surface disappears.In view of this, the thickness of underlying layer 21 needs to belimited such that groove portion 17 is not completely filled up (thatis, the crystal surface does not disappear). Preferably, the thicknessof underlying layer 21 is from 0.01 μm to 2 μm.

The depth Z of groove portion 17 may be from 0.5 μm to 20 μm, and morepreferably from 0.5 μm to 8 μm. If the depth of groove portion 17 isless than 0.5 μm, although crystal surface 16 appears after commencementof growth of underlying layer 21, groove portion 17 is filled up in thecourse of growth and crystal surface 16 cannot be maintained, which isnot preferable. As the groove becomes deeper, surface flatnessdeteriorates, and therefore, the groove depth Z is preferably 8 μm orless. On the other hand, if the depth Z of groove portion 17 exceeds 20μm, cracking of the wafer occurs in the production process of thenitride semiconductor laser device, which is not preferable. Byselecting the depth of groove portion 17 in this manner, groove portion17 is prevented from being filled up by nitride semiconductor (theportions represented by reference numeral 21 and 11 in FIG. 4, describedlater), and consequently, the crystal surface is prevented fromdisappearing.

The width of groove portion 17 is preferably from 5 μm to 100 μm. Thisis because if the width of groove portion 17 is less than 5 μm, grooveportion 17 is filled up by nitride semiconductor (underlying layer) andthe crystal surface disappears, and thus improvement of flatness anduniformity of the nitride semiconductor layers cannot be expected. Ifthe width of groove portion 17 exceeds 100 μm, the number of nitridesemiconductor light-emitting devices (chips) obtained from one wafer isreduced, which is not preferable.

2) Formation of Nitride Light-Emitting Device

As an example of the nitride light-emitting device, a nitridesemiconductor laser device will be described. FIG. 4 is a schematiccross sectional view of a part of a bar provided with a nitridesemiconductor laser device of this embodiment. In this embodiment, asnitride semiconductor substrate 10, an n-type GaN substrate with the Csurface being the main surface is used. On the n-type GaN substrate(nitride semiconductor substrate 10), groove portions 17 and hillportion 18 are formed in the direction parallel to the <1-100>direction. The cross sectional shapes of the groove portions and hillportion are rectangular. FIG. 5 is a schematic cross sectional view ofthe vicinity of the hill portion shown in FIG. 4, and the structure oflayer 11 of light-emitting-device structure made of nitridesemiconductor is shown. Layer 11 of light-emitting-device structure madeof nitride semiconductor shown in FIG. 4 corresponds to the nitridesemiconductor layers represented by reference numeral 22 to 30 shown inFIG. 5.

Next, a method for producing the nitride semiconductor laser device ofthis embodiment will be described. First, referring to FIGS. 3 and 4,n-type GaN substrate 10 having stripe groove portions 17 is prepared. Todescribe specifically, on the upper surface of n-type GaN substrate 10,SiO₂, SiN_(x), or the like is deposited (not shown). It should be notedthat although SiO₂ is used in this embodiment, this is not to berestrictive and another dielectric film or the like can be used. Next,on the SiO₂ film, a resist material is applied, and by using a generallithography technique, a stripe-shaped resist-mask pattern is formed inthe <1-100> direction (not shown). Next, by using the RIE (Reactive IonEtching) technique or the like, etching is carried out over SiO₂ and theupper surface of n-type GaN substrate 10, thereby forming grooveportions 17. Then, by using an etchant such as HF (hydrofluoric acid),SiO₂ is removed, thereby preparing n-type GaN substrate 10 on whichstripe groove portions 17 are formed in the direction parallel to the<1-100> direction. In this embodiment, the width of each of grooveportions 17 was 5 μm, the width of hill portion 18 was 395 μm, and thedepth Z of groove portions 17 was 5 μm.

It should be noted that although in this embodiment, the RIE techniquewas used in forming groove portions 17 by subjecting the upper surfaceof n-type GaN substrate 10 to etching, this method is not to berestrictive and the wet etching technique or the like can be used. Inaddition, the cross sectional shape of each of groove portions 17 (orhill portion 18) can be rectangular, or a forward tapered shape suchthat the width of the opening portion of each of groove portions 17 iswider than the width of the bottom surface portion, or inversely, aninverse tapered shape such that the opening portion of each of grooveportions 17 is narrower than the width of the bottom surface portion.

Next, by carrying out treatment in accordance with the conditionsdescribed in (1) above, underlying layer 21 (e.g., an n-type GaNunderlying layer) is formed on substrate 10. Over substrate 10 on whichunderlying layer 21 is formed, a nitride semiconductor laser device isprepared by using the MOCVD method or the like.

As shown in FIG. 5, the nitride semiconductor laser device includesn-type GaN 10, underlying layer 21 made of nitride semiconductor of 0.2μm thick, n-type A_(0.062)Ga_(0.938)N first cladding layer 22 (1.5 μm),n-type Al_(0.1)Ga_(0.9)N second cladding layer 23 (0.2 μm), n-typeAl_(0.062)Ga_(0.938)N third cladding layer 24 (0.1 μm), n-type GaNguiding layer 25 (0.1 μm), active layer 26 of multi-quantum wellstructure composed of four 8 nm-thick In_(0.1)Ga_(0.99)N barrier layersand three 4 nm-thick In_(0.1)Ga_(0.9)N well layers, p-typeAl_(0.3)Ga_(0.7)N carrier blocking layer 27 of 20 nm thick, p-type GaNlight guiding layer 28 of 0.1 μm thick, p-type Al_(0.1)Ga_(0.9)Ncladding layer 29 of 0.5 μm thick, and p-type GaN contact layer 30 of0.1 μm thick.

Here the lower cladding layer of this embodiment is an n-type claddinglayer, and specifically, the cladding layers indicated by referencenumeral 22 to 24 in FIG. 5. Also, the upper cladding layer of thisembodiment is a p-type cladding layer, and specifically, the claddinglayer indicated by reference numeral 29 in FIG. 5.

Next, as shown in FIG. 4, on the surface of layer 11 oflight-emitting-device structure made of nitride semiconductor, ridgestripe portion 12 as a current constricting portion and insulation film13 (e.g., SiO₂) for the purpose of current constriction provided in amanner of sandwiching ridge stripe portion 12 are formed.

Ridge stripe portion 12 is generally formed by using thephotolithography technique and the dry etching technique such thatetching is carried out to leave a stripe shape from the outermostsurface (p-type GaN contact layer 30) of layer 11 oflight-emitting-device structure to the halfway of the thickness ofp-type Al_(0.1)Ga_(0.9)N cladding layer 29. The width of the stripe isfrom 1 to 3 μm, preferably from 1.3 to 2 μm. The distance from theinterface between p-type GaN light guiding layer 28 and p-typeAl_(0.1)Ga_(0.9)N cladding layer 29 to the etching bottom surface isfrom 0.1 to 0.4 μm. Other than SiO₂, insulation film 13 can be an oxideor nitride of silicon, titanium, zirconium, tantalum, aluminum, or thelike.

Next, on the exposed portion of striped-shaped p-type GaN contact layer30, which is left unetched, and on insulation film 13, p-electrode 14 isformed in the order of Pd/Mo/Au. Other than this, p-electrode 14 can bePd/Pt/Au, Pd/Au/, Ni/Au, or the like.

Next, by carrying out polishing or etching from the back surface ofn-type GaN 10, the thickness of the wafer is reduced to 80-200 μm. Then,as n-electrode 15, Hf/Al is formed on the back surface of n-type GaN 10from the side near n-type GaN 10. The material used for n-type electrode15 is not limited to Hf/Al, and Hf/Al/Mo/Au, Hf/Al/Pt/Au, Hf/Al/W/Au,Hf/Au, Hf/Mo/Au, or the like can be used. These materials can be suchelectrode materials that Hf is replaced with Ti or Zr.

As shown in FIG. 4, n-electrode 15 can be formed on each nitridesemiconductor laser device, or n-electrode 15 can be formed on theentire back surface of n-type GaN substrate 10 (or on the back surfaceof the wafer).

After ridge stripe portion 12, p-electrode 14, and n-electrode 15 arethus formed, the wafer is cleaved in the direction perpendicular to the<1-100> direction (see FIG. 4), in which ridge stripe portion 12 isformed. Thus, an optical cavity is formed. In this embodiment, awaveguiding-type Fabry-Perot resonator with a cavity length of 600 μmwas prepared. It should be noted that the length of the optical cavityis not limited to 600 μm, and the length can be within the range of from300 μm to 1000 μm.

As described above, by carrying out the step of forming an opticalcavity by cleaving the wafer, a bar shape is formed. The bar has amultiplicity of nitride semiconductor laser structures shown in FIG. 4formed side by side. The formed cleaved facet(s) corresponds to the{1-100} surface of the nitride semiconductor crystal. Cleavage iscarried out such that a ruled line is drawn on the entire back surfaceof the wafer with a diamond cutter and suitable force is applied on thewafer. It is possible that a line is drawn only on a part of the wafer,e.g., the edge portion of the wafer, with a diamond cutter, and this canact as the starting point of cleavage, or the optical cavity can beformed by etching.

The position in which the ridge stripe portion 12 is formed is notparticularly specified insofar as the position is a flat region on hillportion 18, and more preferably, the position is 20 μm or more apartfrom the edge of the hill portion. In this embodiment, ridge stripeportion 12 was formed in the center of hill portion 18.

After two cleaved facet(s) are thus formed at the front and back of thewaveguiding-type Fabry-Perot resonator, on both the cleaved facet(s),dielectric films made of SiO₂ and TiO₂ with 70% reflectivity arealternately deposited, thereby forming dielectric multi-layeredreflective films. Of the two formed cleaved facet(s), one is made alaser outputting surface, and for example, the reflectivity of thedielectric multi-layered reflective film formed on the correspondingcleaved facet(s) is made 5%. The other cleaved facet(s) is made a laserreflecting surface, and for example, the reflectivity of the dielectricmulti-layered reflective film formed on the corresponding cleavedfacet(s) is made 95%. It should be noted that the reflectivity is notlimited to these values. Also, the dielectric film material is notlimited to SiO₂/TiO₂, and for example, SiO₂/Al₂O₃ or the like can beused.

Next, by dividing the bar in which a multiplicity of nitridesemiconductor laser devices are formed side by side in the directionparallel to ridge stripe portion 12, separate nitride semiconductorlaser devices (chips) are obtained. Here the obtained bar is placed on astage with the back surface side of the wafer facing upwards, thepositions to be flawed are aligned by using an optical microscope, andruled lines are drawn on the back surface of the bar with a diamondcutter. Then, suitable force is applied on the bar to divide it alongthe ruled lines, thereby forming nitride semiconductor laser devices(chips). This method is called the scribing method.

Other than the scribing method, the chip dividing step can be carriedout by, for example, the dicing method, in which flawing or cutting iscarried out by using a wire saw or thin blade, the laser scribingmethod, in which heating is carried out by radiation of laser light froman excimer laser or the like and subsequently rapid cooling is carriedout to form a crack at the radiation portion and the crack acts as ascribed line, the laser ablation method, in which laser light of highenergy density is radiated and evaporated to carry out groove cuttingtreatment, or the like.

By the chip dividing step described above, the nitride semiconductorlaser devices formed over hill portions 18 are divided into separatechips as shown in FIG. 4. The width of each divided nitridesemiconductor laser device is 400 μm. In this embodiment, on n-type GaNsubstrate 10, groove portions 17 and hill portion 18 were formed every400 μm, and cleavage was carried out at the vicinity of the center ofgroove portion 17 and along dividing line 41 shown in FIG. 4. It shouldbe noted that while in this embodiment division was carried out atgroove portion 17, division can exclude groove portion 17 and be carriedout only at hill portion 18, resulting in a nitride semiconductor laserdevice that does not include groove portion 17. Although the position ofdividing line 41 is not limited to those described above, the positionis preferably at least 20 μm or more apart from ridge stripe portion 12.

The wafer of this embodiment prepared in the method described above wascleaved in the <11-20> direction, and the cross section of the wafer wasobserved with an SEM (Scanning Electron Microscope). Groove portion 17,formed on the substrate, was not filled up with underlying layer 21 orlayer 11 of light-emitting-device structure but formed a concave. Inthis concave, crystal surfaces formed in the vicinities of the sidesurfaces of the groove portion existed, and each of the crystal surfaceswas the {11-22} surface. As described above, when, after accumulatingunderlying layer 21 over n-type GaN substrate 10 on which grooveportions 17 and hill portion 18 were formed and forming crystal surfaces16, layer 11 of light-emitting-device structure made of nitridesemiconductor was accumulated, the flatness of the surface in thevicinity of the center of the hill portion was satisfactory and thethickness of layer 11 of light-emitting-device structure made of nitridesemiconductor was uniform.

When measuring for cracks in the wafer (nitride semiconductor laserdevice) was carried out, the number of cracks per area of 1 cm² was from0 to 1 when the groove depth Z was from 0.5 μm to 8 μm. When the groovedepth Z was less than 0.5 μm, there was such a tendency that the numberof cracks increased as Z became less. This is because the grooves arefilled up immediately in the course of growth and the crystal surfacecannot be maintained. If the groove depth Z is greater than 8 μm,although the number of cracks remains from 0 to 1, surface flatnessdeteriorates. Incidentally, in a nitride semiconductor laser device ofthe prior art, 3 to 6 cracks occurred per area of 1 cm², and theflatness of the surface was not satisfactory and the thickness of layer11 of light-emitting-device structure was not uniform. By using thepresent invention in such a manner, cracks were inhibited, and at thesame time, the uniformity of surface flatness and thickness of layer 11of light-emitting-device structure was realized, resulting in animproved yield rate.

While in this embodiment an n-type GaN underlying layer is used as theunderlying layer 21, such an underlying layer of AlGaN can be used thatthe composition ratio of Al is as described in (1) above. As the n-typedopant, Si is preferably used, and the Si density is preferably from5×10¹⁷ cm³ to 8×10¹⁸ cm³.

While active layer 26 of multi-quantum well structure of this embodimenthas such a structure that starts with a barrier layer and ends with abarrier layer, such a structure can be used that starts with a welllayer and ends with a well layer. The number of the well layers is notlimited to three as described above and can be ten or less, because thethreshold value current density is low and room-temperature continuousoscillation is realized. In particular, the number of the well layers ispreferably from two to six because the threshold value current densityis low. It is possible to add Al in active layer 26 of multi-quantumwell structure.

While in this embodiment a Fabry-Perot resonator is provided, thissystem is not to be restrictive, and such a nitride semiconductor lasercan be used that uses another feedback system such as a distributedfeedback (DFB) laser in which the grating is provided inside the currentinjection region, and a distributed bragg reflector (DBR) laser in whichthe grating is provided outside the current injection region.

By using the present invention, a reduction in cracking, improvement ofsurface flatness, and uniformity of thickness of the nitridesemiconductor light-emitting device are realized, thereby improving theyield rate of the nitride semiconductor light-emitting device. Thenitride semiconductor light-emitting device according to the presentinvention refers to, for example, a nitride semiconductor laser diode, anitride semiconductor light-emitting diode, and the like. Also, thepresent invention is suitable for blue semiconductor lasers mounted inthe optical pickups of optical disk devices.

The embodiment herein described is to be considered in all respects asillustrative and not restrictive. The scope of the invention should bedetermined not by the Embodiments illustrated, but by the appendedclaims, and all changes which come within the meaning and range ofequivalency of the appended claims are therefore intended to be embracedtherein.

1. A method for producing a nitride semiconductor light-emitting devicecomprising the steps of: preparing a nitride semiconductor substratehaving stripe groove portions; forming an underlying layer comprisingnitride semiconductor on the nitride semiconductor substrate includingside walls of the groove portions, in such a manner that the underlyinglayer has a crystal surface in each of the groove portions and thecrystal surface is tilted at an angle of from 53.5° to 63.4° withrespect to a surface of the substrate; and sequentially forming, overthe underlying layer, a lower cladding layer containing Al, an activelayer, and an upper cladding layer containing Al.
 2. The method forproducing a nitride semiconductor light-emitting device according toclaim 1, wherein a composition ratio of Al in the underlying layer islower than a composition ratio of Al in the lower cladding layer.
 3. Themethod for producing a nitride semiconductor light-emitting deviceaccording to claim 2, wherein the composition ratio of Al in theunderlying layer is 5% or lower.
 4. The method for producing a nitridesemiconductor light-emitting device according to claim 1, wherein theunderlying layer comprises GaN.
 5. The method for producing a nitridesemiconductor light-emitting device according to claim 1, wherein thecrystal surface comprises a {11-22} surface.
 6. The method for producinga nitride semiconductor light-emitting device according to claim 1,wherein thickness of the underlying layer is from 0.01 μm to 2 μm. 7.The method for producing a nitride semiconductor light-emitting deviceaccording to claim 1, wherein depth of each of the groove portions isfrom 0.5 μm to 20 μm.
 8. The method for producing a nitridesemiconductor light-emitting device according to claim 1, wherein widthof each of the groove portions is from 5 μm to 100 μm.
 9. A nitridesemiconductor light-emitting device comprising: a nitride semiconductorsubstrate; an underlying layer provided on the nitride semiconductorsubstrate and comprising nitride semiconductor; and a lower claddinglayer containing Al, an active layer, and an upper cladding layercontaining Al, the layers being provided sequentially on the underlyinglayer.
 10. The nitride semiconductor light-emitting device according toclaim 9, wherein a composition ratio of Al in the underlying layer islower than a composition ratio of Al in the lower cladding layer. 11.The nitride semiconductor light-emitting device according to claim 10,wherein the composition ratio of Al in the underlying layer is 5% orlower.
 12. The nitride semiconductor light-emitting device according toclaim 11, wherein the underlying layer comprises GaN.
 13. The nitridesemiconductor light-emitting device according to claim 9, whereinthickness of the underlying layer is from 0.01 μm to 2 μm.